W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
The dedicated channel (DCH) for downlink and uplink and the downlink shared channel (DSCH) have been defined in Release 99 and Release 4. In the following years, the developers recognized that for providing multimedia services—or data services in general—high speed asymmetric access had to be implemented. In Release 5 the high-speed downlink packet access (HSDPA) was introduced. The new high-speed downlink shared channel (HS-DSCH) provides downlink high-speed access to the user from the UMTS Radio Access Network (RAN) to the communication terminals, called user equipments in the UMTS specifications.
Packet Scheduling
Packet scheduling is a radio resource management algorithm used for allocating transmission opportunities and transmission formats to the users admitted to a shared medium. Scheduling may be used in packet based mobile radio networks in combination with adaptive modulation and coding to maximize throughput/capacity by e.g. allocating transmission opportunities to the users in favorable channel conditions. The packet data service in UMTS may be applicable for the interactive and background traffic classes, though it may also be used for streaming services. Traffic belonging to the interactive and background classes is treated as non real time (NRT) traffic and is controlled by the packet scheduler. The packet scheduling methodologies can be characterized by:
Scheduling period/frequency: The period over which users are scheduled ahead in time.
Serve order: The order in which users are served, e.g. random order (round robin) or according to channel quality (C/I or throughput based).
Allocation method: The criterion for allocating resources, e.g. same data amount or same power/code/time resources for all queued users per allocation interval.
The packet scheduler for uplink is distributed between Radio Network Controller (RNC) and user equipment in 3GPP UMTS R99/R4/R5. On the uplink, the air interface resource to be shared by different users is the total received power at a Node B, and consequently the task of the scheduler is to allocate the power among the user equipment(s). In current UMTS R99/R4/R5 specifications the RNC controls the maximum rate/power a user equipment is allowed to transmit during uplink transmission by allocating a set of different transport formats (modulation scheme, code rate, etc.) to each user equipment.
The establishment and reconfiguration of such a TFCS (transport format combination set) may be accomplished using Radio Resource Control (RRC) messaging between the serving RNC (S-RNC) and the user equipment. The user equipment is allowed to autonomously choose among the allocated transport format combinations based on its own status e.g. available power and buffer status.
In current UMTS R99/R4/R5 specifications there is no control on time imposed on the uplink user equipment transmissions. The scheduler may e.g. operate on transmission time interval basis. In UMTS Release 6 for enhanced uplink dedicated channels (E-DCHs) the scheduler may operate with higher scheduling frequency with respect to legacy channels (on a short TTI—e.g. 2 ms—basis). This may impose certain serve order of the terminals while maintaining resources on allocation by a Node B on the basis of the noise rise.
UMTS Architecture
The high level R99/4/5 architecture of Universal Mobile Telecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, available from http://www.3gpp.org). The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
In the sequel two different architectures will be discussed. They are defined with respect to logical distribution of functions across network elements. In actual network deployment, each architecture may have different physical realizations meaning that two or more network elements may be combined into a single physical node.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the user equipments. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called Iu connection with the Core Network (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supports the Serving RNS (S-RNS) 301 by providing radio resources as shown in FIG. 3.
Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used.
Radio Mobility Management
Radio Mobility Management for Rel99/4/5 UTRAN
Before explaining some procedures connected to mobility management, some terms frequently used in the following are defined first.
A radio link may be defined as a logical association between single UE and a single UTRAN access point. Its physical realization comprises radio bearer transmissions.
A handover may be understood as a transfer of a UE connection from one radio bearer to another (hard handover) with a temporary break in connection or inclusion/exclusion of a radio bearer to/from UE connection so that UE is constantly connected UTRAN (soft handover). Soft handover is specific for networks employing Code Division Multiple Access (CDMA) technology. Handover execution may controlled by S-RNC in the mobile radio network when taking the present UTRAN architecture as an example.
The active set associated to a UE comprises a set of radio links simultaneously involved in a specific communication service between UE and radio network. An active set update procedure may be employed to modify the active set of the communication between UE and UTRAN, for example during soft-handover. The procedure may comprise three functions: radio link addition, radio link removal and combined radio link addition and removal. The maximum number of simultaneous radio links is set to eight. New radio links are added to the active set once the pilot signal strengths of respective base stations exceed certain threshold relative to the pilot signal of the strongest member within active set.
A radio link is removed from the active set once the pilot signal strength of the respective base station exceeds certain threshold relative to the strongest member of the active set. Threshold for radio link addition is typically chosen to be higher than that for the radio link deletion. Hence, addition and removal events form a hysteresis with respect to pilot signal strengths.
Pilot signal measurements may be reported to the network (e.g. to S-RNC) from UE by means of RRC signaling. Before sending measurement results, some filtering is usually performed to average out the fast fading. Typical filtering duration may be about 200 ms contributing to handover delay. Based on measurement results, the network (e.g. S-RNC) may decide to trigger the execution of one of the functions of active set update procedure (addition/removal of a Node B to/from current active set).
Radio Mobility Management for E-DCH
In Release 6 of UMTS it is presently foreseen to support soft handover for the E-DCH transmissions. However the active sets for a legacy DCH (Dedicated CHannel) and E-DCH are generally different.
Common and Dedicated Measurements on the Iub Interface
Common and dedicated measurement procedures are commonly initiated by sending a common/dedicated measurement initiation message from a C-RNC to a connected Node B using the Node B Control Port. Upon reception, the Node B initiates the requested measurement according to the parameters given in the request. The addressed Node B sends a common/dedicated measurement report to the C-RNC in response to the initiation request. The request sent by the C-RNC as well as the reports sent by the addresses Node B comprise Measurement ID IE having a Measurement ID set in order to allow an association between a measurement request and the corresponding reports.
Common measurement procedures on the Iub interface are used for measurements on common resource in Node B. Analogously, dedicated measurement procedures on the Iub interface are used for measurements on dedicated resource in Node B Both types of procedures may be configured for periodic, event-triggered and immediate type of reporting (see 3GPP TS 25.433: “UTRAN Iub Interface NBAP Signaling”, version 6.1.0).
Enhanced Uplink Dedicated Channel (E-DCH)
Uplink enhancements for Dedicated Transport Channels (DTCH) are currently studied by the 3GPP Technical Specification Group RAN (see 3GPP TR 25.896: “Feasibility Study for Enhanced Uplink for UTRA FDD (Release 6)”, available at http://www.3gpp.org). Since the use of IP-based services become more important, there is an increasing demand to improve the coverage and throughput of the RAN as well as to reduce the delay of the uplink dedicated transport channels. Streaming, interactive and background services could benefit from this enhanced uplink.
One enhancement is the usage of adaptive modulation and coding schemes (AMC) in connection with Node B controlled scheduling, thus an enhancement of the Uu interface. In the existing R99/R4/R5 system the uplink maximum data rate control resides in the RNC. By relocating the scheduler in the Node B the latency introduced due to signaling on the interface between RNC and Node B may be reduced and thus the scheduler may be able to respond faster to temporal changes in the uplink load. This may reduce the overall latency in communications of the user equipment with the RAN. Therefore Node B controlled scheduling is capable of better controlling the uplink interference and smoothing the noise rise variance by allocating higher data rates quickly when the uplink load decreases and respectively by restricting the uplink data rates when the uplink load increases. The coverage and cell throughput may be improved by a better control of the uplink interference.
Another technique, which may be considered to reduce the delay on the uplink, is introducing a shorter TTI (Transmission Time Interval) length for the E-DCH compared to other transport channels. A transmission time interval length of 2 ms is currently investigated for use on the E-DCH, while a transmission time interval of 10 ms is commonly used on the other channels. Hybrid ARQ, which was one of the key technologies in HSDPA, is also considered for the enhanced uplink dedicated channel. The Hybrid ARQ protocol between a Node B and a user equipment allows for rapid retransmissions of erroneously received data units, and may thus reduce the number of RLC (Radio Link Control) retransmissions and the associated delays. This may improve the quality of service experienced by the end user.
To support enhancements described above, a new MAC sub-layer is introduced which will be called MAC-e in the following. The entities of this new sub-layer, which will be described in more detail in the following sections, may be located in user equipment and Node B. On user equipment side, the MAC-e performs the new task of multiplexing upper layer data (e.g. MAC-d) data into the new enhanced transport channels and operating HARQ protocol transmitting entities.
E-DCH MAC Architecture—UE side
The MAC-e entity at the UE is depicted in more detail in FIG. 4. There are M different data flows (MAC-d) carrying data packets from different applications to be transmitted from UE to Node B. These data flows can have different QoS requirements (e.g. delay and error requirements) and may require different configuration of HARQ instances. Each MAC-d flow represents a logical unit to which specific physical channel (e.g. gain factor) and HARQ (e.g. maximum number of retransmissions) attributes can be assigned.
Further, MAC-d multiplexing is supported for an E-DCH, i.e. several logical channels with different priorities may be multiplexed onto the same MAC-d flow. Therefore the data from one MAC-d flow can be fed into different Priority Queues. The selection of an appropriate transport format for the transmission of data on E-DCH is done in the TF Selection entity which represents a function entity. The transport format selection is based on the available transmit power, priorities, e.g. logical channel priorities, and associated control signaling (HARQ and scheduling related control signaling) received from a Node B. The HARQ entity handles the retransmission functionality for the user. One HARQ entity supports multiple HARQ processes. The HARQ entity handles all HARQ related functionalities required. MAC-e entity receives scheduling information from Node B (network side) via Layer-1 signaling as shown in FIG. 4.
E-DCH MAC Architecture—UTRAN side
In soft handover operation it may be assume that the MAC-e entities are distributed across Node B (MAC-eb) and S-RNC (MAC-es) on UTRAN side. The scheduler in Node B chooses the active users among these entities and performs rate control through a commanded rate, suggested rate or TFC threshold that limits the active user (UE) to a subset of the TCFS. Every MAC-e entity corresponds to a user (UE). In FIG. 5 the Node B's MAC-e architecture is depicted in more detail. It can be noted that each HARQ Retransmission entity is assigned certain amount of the soft buffer memory for combining the bits of the packets from outstanding retransmissions. Once a packet is received successfully, it is forwarded to the reordering buffer providing the in-sequence delivery to upper layer.
It may be assumed that the reordering buffer resides in S-RNC during soft handover. In FIG. 6 the S-RNC's MAC-e architecture which comprises the reordering buffer of the corresponding user (UE) is shown. The number of reordering buffers is equal to the number of data flows in the corresponding MAC-e entity on UE side. Data and control information is sent from all Node Bs within active set to S-RNC during soft handover.
It should be noted that the required soft buffer size depends on the used HARQ scheme, e.g. an HARQ scheme using incremental redundancy (IR) requires more soft buffer than one with chase combining (CC).
Reordering Function
Several data flows may be multiplexed into one MAC-e PDU on UE side to improve frame fill efficiency. If RLC protocol is configured to work in the acknowledged (AM) mode, in-sequence delivery of RLC PDUs to the RLC entity on the network side is required in order to avoid unnecessary detection of losses and retransmissions on the RLC level.
The operation of reordering function is determined by two major parameters, which are for the purpose of this report termed as Receiver Window and Reordering Release Timer, as illustrated in the FIG. 8. The Receiver window sets up an upper bound for acceptable maximum data rate on the uplink. Whenever a PDU with TSN being larger than current upper edge of the window enters reordering buffer, Receiver Window is moved in the direction of increasingly larger TSNs and the PDUs remaining out of it are immediately forwarded to the RLC receiving entity. The window is also moved in the same direction after expiry of the Reordering Release timer thus allowing for detection of gaps by RLC receiving entity.
The details of the reordering function have not yet been standardized, but it is all probability that the two major parameters will be subject to semi-static configuration by S-RNC (legacy architecture).
E-DCH—Node B Controlled Scheduling
Node B controlled scheduling is one of the technical features for E-DCH which is foreseen to enable more efficient use of the uplink power resource in order to provide a higher cell throughput in the uplink and to increase the coverage. The term “Node B controlled scheduling” denotes the possibility for the Node B to control, within the limits set by the RNC, the set of TFCs from which the UE may choose a suitable TFC. The set of TFCs from which the UE may choose autonomously a TFC is in the following referred to as “Node B controlled TFC subset”.
The “Node B controlled TFC subset” is a subset of the TFCS configured by RNC as seen in FIG. 7. The UE selects a suitable TFC from the “Node B controlled TFC subset” employing the Rel5 TFC selection algorithm. Any TFC in the “Node B controlled TFC subset” might be selected by the UE, provided there is sufficient power margin, sufficient data available and TFC is not in the blocked state. Two fundamental approaches to scheduling UE transmission for the E-DCH exist. The scheduling schemes can all be viewed as management of the TFC selection in the UE and mainly differs in how the Node B can influence this process and the associated signaling requirements.
Node B Controlled Rate Scheduling
The principle of this scheduling approach is to allow Node B to control and restrict the transport format combination selection of the user equipment by fast TFCS restriction control. A Node B may expand/reduce the “Node B controlled subset”, which user equipment can choose autonomously on suitable transport format combination from, by Layer-1 signaling. In Node B controlled rate scheduling all uplink transmissions may occur in parallel but at a rate low enough such that the noise rise threshold at the Node B is not exceeded. Hence, transmissions from different user equipments may overlap in time. With Rate scheduling a Node B can only restrict the uplink TFCS but does not have any control of the time when UEs are transmitting data on the E-DCH. Due to Node B being unaware of the number of UEs transmitting at the same time no precise control of the uplink noise rise in the cell may be possible (see 3GPP TR 25.896: “Feasibility study for Enhanced Uplink for UTRA FDD (Release 6)”, version 1.0.0, available at http://www.3gpp.org).
Two new Layer-1 messages are introduced in order to enable the transport format combination control by Layer-1 signaling between the Node B and the user equipment. A Rate Request (RR) may be sent in the uplink by the user equipment to the Node B. With the RR the user equipment can request the Node B to expand/reduce the “Node controlled TFC Subset” by one step. Further, a Rate Grant (RG) may be sent in the downlink by the Node B to the user equipment. Using the RG, the Node B may change the “Node B controlled TFC Subset”, e.g. by sending up/down commands. The new “Node B controlled TFC Subset” is valid until the next time it is updated.
Node B Controlled Rate and Time Scheduling
The basic principle of Node B controlled time and rate scheduling is to allow (theoretically only) a subset of the user equipments to transmit at a given time, such that the desired total noise rise at the Node B is not exceeded. Instead of sending up/down commands to expand/reduce the “Node B controlled TFC Subset” by one step, a Node B may update the transport format combination subset to any allowed value through explicit signaling, e.g. by sending a TFCS indicator (which could be a pointer).
Furthermore, a Node B may set the start time and the validity period a user equipment is allowed to transmit. Updates of the “Node B controlled TFC Subsets” for different user equipments may be coordinated by the scheduler in order to avoid transmissions from multiple user equipments overlapping in time to the extent possible. In the uplink of CDMA systems, simultaneous transmissions always interfere with each other. Therefore by controlling the number of user equipments, transmitting simultaneously data on the E-DCH, Node B may have more precise control of the uplink interference level in the cell. The Node B scheduler may decide which user equipments are allowed to transmit and the corresponding TFCS indicator on a per transmission time interval (TTI) basis based on, for example, buffer status of the user equipment, power status of the user equipment and available interference Rise over Thermal (RoT) margin at the Node B.
Two new Layer-1 messages are introduced in order to support Node B controlled time and rate scheduling. A Scheduling Information Update (SI) may be sent in the uplink by the user equipment to the Node B. If user equipment finds a need for sending scheduling request to Node B (for example new data occurs in user equipment buffer), a user equipment may transmit required scheduling information. With this scheduling information the user equipment provides Node B information on its status, for example its buffer occupancy and available transmit power.
A Scheduling Grant (SG) may be transmitted in the downlink from a Node B to a user equipment. Upon receiving the scheduling request the Node B may schedule a user equipment based on the scheduling information (SI) and parameters like available RoT margin at the Node B. In the Scheduling Grant (SG) the Node B may signal the TFCS indicator and subsequent transmission start time and validity period to be used by the user equipment.
The usage of either rate or time and rate scheduling is of course restricted by the available power, as the E-DCH will have to co-exist with a mix of other transmissions by that UE and other UEs in the uplink. The co-existence of the different scheduling modes may provide flexibility in serving different traffic types. For example, the applications demanding lower data rates may be sent over E-DCH in rate controlled mode while the applications demanding higher data rate may be sent over E-DCH in time and rate controlled mode.
Serving Node B and its Role in Node B Controlled Scheduling
The following section will briefly summarize the scheduling operation from a radio interface Layer-2 perspective. The Node B controlled scheduling is based on uplink and downlink control together with a set of rules on how the UE behaves with respect to this signaling. On the downlink, a resource indication (scheduling grant) is required to indicate to the UE the maximum amount of uplink resources it may use.
Node B Controlled Scheduling for Guaranteed Bit-rate Traffic
Guaranteed Bit-rate traffic is supported by allowing non-scheduled and scheduled data transmissions.
For non-scheduled data transmissions a guaranteed bit-rate for a MAC-d flow or logical channel may be supported. Non-scheduled transmissions mean that there can be autonomous transmissions without a scheduler in the Node B granting the transmissions. Generally, the S-RNC decides on whether traffic is transmitted in a non-scheduled mode and reports this decision to a respective UE and to the Node-Bs in communication with the UE. The respective Node-Bs should reserve sufficient amount of resources based on statistical multiplexing gains for non-scheduled data transmissions. The mechanism can be used e.g. for Guaranteed Bit-rate delay-sensitive applications such as voice and/or for signaling radio bearers.
For scheduled data transmissions, a guaranteed bit-rate for a UE is supported. The respective guaranteed bit-rate value is provided by the S-RNC to a Node B, and the scheduler shall act upon it this configuration parameter. The mechanism can be advantageously used e.g. for Guaranteed Bit-rate non-delay-sensitive applications such as streaming.
Scheduling Grants
Scheduling grants can be sent once per TTI or slower. There are two types of grants: absolute grants and relative grants. The absolute grants provide an absolute limitation of the maximum amount of UL resources the UE may use. The relative grants increase or decrease the resource limitation compared to the previously used value.
When considering soft handover (SHO) operation of E-DCH, serving and non-serving Node Bs may be defined. The serving Node B may be defined as the Node B controlling the serving cell of the UE in soft handover. It is important to note that absolute grants may be sent by serving Node B only, while relative grants may be sent by both a serving and non-serving Node B. The cell through which a UE receives absolute grants is referred to the serving cell. Further, the Node B controlling the serving cell is referred to as the serving Node B or S-Node B.
As indicated above, absolute scheduling grants are sent through the serving cell and are valid for one UE, for a group of UEs or for all UEs. Further, the absolute grants may have an associated duration of validity.
Relative scheduling grants (updates) are sent by the serving and non-serving Node-Bs as a complement to absolute grants. The relative grant from the serving Node-B can take one of the three values: “UP”, “HOLD” or “DOWN”. Further, relative grants from the non-serving Node-B can take one of the two values: “HOLD” or “DOWN”. The “DOWN” command corresponds to an “overload indicator”.
The UE behavior is defined by the way in which absolute/relative grants are processed in the mobile terminal. One exemplary operation of a UE receiving scheduling grants may be as follows.
The UE maintains a “serving Node-B grant”, which corresponds to the last absolute grant received from the E-DCH serving cell which has then been modified, every TTI, by serving Node-B relative grants. This operation is independent of the relative grants received from the non-serving Node-Bs. If at least one non-serving Node-B indicates “DOWN”, the UE may degrade the current used bit-rate by a pre-defined offset. The offset may be dependant on the bit-rate.
The calculation of the pre-defined offset is implementation dependent. E.g. the offset may be function of the measured CPICH power on the overloaded cells in relation to the measured CPICH power on the serving cell.
When no more “DOWN” is received from any non-serving Node-Bs the UEs gradually increases its current bit-rate, by another pre-defined offset until it reaches maintained “serving Node-B grant”. The offset may be dependant on the bit-rate. Once the “serving Node-B grant” has been reached, and as long as no “DOWN” is received from any non-serving Node-Bs, the UE follows serving Node-B.
The common denominator for the present and other considered UE behaviours is that the upper-most limit for uplink data rate by the UE is set by serving Node B and that the upper limit can be temporarily constrained by non-serving Node Bs. As in UMTS Release 99 also for E-DCH in Release 6, a gain factor denoting the power offset from the DPCCH is calculated by a UE or is explicitly signaled from the UTRAN for each TFC (transport format combination) used for uplink data transmission.
Currently there is a so-called “boosted mode” and “nominal mode” under discussion within 3GPP. The “boosted mode” should be used for the transmission of very delay critical data. The transmission boost is achieved by some additional gain factor (power offset) for the uplink data transmission. The gain factor for “nominal” mode is the calculated or explicitly signalled gain factor for “boosted” mode as described before. It is clear that UEs in “boosted” mode contribute more significantly to the rise over thermal (RoT) than UEs in nominal mode.
When considering the current scheme, it is obvious that the influence of said temporary limitation upon guaranteed bit-rate traffic depends on active set Update criteria, UE mode with respect to gain factors (boosted, nominal) and settings for said offsets. active set Update Criteria are a matter of network implementation and are not expected to contribute resolutely to differentiation of UEs. On the other side, the UE gain factor and offset settings (which may depend on the required bit-rate) may be significantly different amongst various UEs thus implying that aggregated measurements for a cell convey insufficient amount of information. Therefore in certain scenarios dedicated measurements are clearly advantageous when compared against common (aggregated) Layer-2 measurements.
Definition of the Costly UEs
Each of the TFs used for uplink transmissions on the E-DCH may be associated to certain amount of the noise rise in the Node Bs in the active set. Therefore, each UE may be associated to a certain cost factor which reflects the noise rise caused by the UE within the cell.
An exemplary mapping between TFs in the TF set of the UE may be found in the table below.
TFCCost011225354658610
It should be noted that the gain factors of the UEs contribute to the cost of UEs as well.
Functionality Split for E-DCH
When transmitting uplink data via an E-DCH, the data channel is commonly terminated in the S-RNC. However, especially in a soft handover scenario of a mobile terminal the uplink data may be provided from the UE via a Node B and a C-RNC to the S-RNC. In this case the following functional split of network elements may be provided. The C-RNC may be defined as a network element having ownership over resources of Radio Network Subsystem (RNS), while S-RNC may be defined as a network element terminating user-specific functions (e.g. reordering) on the Radio Access Network side.
Node BC-RNCS-RNCAdmission ControlXCongestion ControlXXReorderingXSchedulingXCell Specific uplink resourceXcontrol
The purpose of the admission control is to admit or deny new users, new radio access bearers or new radio links (for example due to handover). The admission control should try to avoid overload situations and base its decisions on interference and resource measurements. The admission control is employed at for example initial UE access, RAB assignment/reconfiguration and at handover. These cases may give different answers depending on priority and situation.
Commonly, the Admission Control function based on uplink interference and downlink power is located in the Controlling RNC. The Serving RNC is performing admission control towards the Iu interface.
The task of congestion control is to monitor, detect and handle situations when the system is reaching a near overload or an overload situation with the already connected users. This means that some part of the network has run out, or will soon run out of resources. The congestion control should then bring the system back to a stable state as seamless as possible.
The scheduling and reordering functionality provided by UMTS have been discussed above.
E-DCH Configuration
Cell-Level E-DCH Configuration
Presently the E-DCH may be configured with respect to “Total power available for E-DCH” when the Node B schedules UEs in the cell so that the measured total E-DCH power does not exceed the signaled Total Power for E-DCH. Secondly, an E-DCH may be configured with respect to “Target/Limit of Total UL power” when the Node B schedules the E-DCH UEs in the cell so that the measured total UL power does not exceed the signaled Target of the Total UL power. Finally, E-DCH may be configured with “Target/Limit of Total UL power” with respect to “Total power available for E-DCH” which is a combination of the previous two ways of configuration.
For each of the three ways of cell level configuration, an E-DCH may be configured per MAC-d flow as described in e.g. in the copending European patent application no. EP 04 023 418.9.
An exemplary transport channel protocol model for E-DCH without Iur mobility is shown in the FIG. 9. It is still unclear whether the E-DCH frame protocol (FP) is terminated in the C-RNC or S-RNC in the case of Iur mobility. An exemplary transport channel protocol model for E-DCH assuming Iur mobility is shown in the FIG. 10. For uplink transmission without Iur mobility (i.e. S-RNC and C-RNC are coincident) the provided bit-rate (per Node B or after macro diversity combining) may be measured in the RNC.
However, in the case of Iur mobility (i.e. S-RNC and C-RNC are not coincident), it is not possible to measure the provided bit-rate for individual uplink transmissions on the E-DCH in the C-RNC, if E-DCH FP is not terminated in the C-RNC as illustrated in the FIG. 10. If E-DCH FP was terminated in the C-RNC, it would be possible to measure a provided bit-rate per Node B but not a provided bit-rate after micro diversity combining.
QoS Classes and Attributes
The nature of the information to be transmitted has a strong influence on the way this information should be transmitted. For instance, a voice call has completely different characteristics than a browsing session (internet). In general, applications and services can be divided into different groups, depending on how they are considered. Four different classes of services have been identified in UMTS and the table below lists their respective characteristics and foreseen use cases.
Traffic classConversationalInteractive classBackgroundclassStreaming classInteractive bestBackground bestconversational RTstreaming RTefforteffortFundamentalPreserve timePreserve timeRequest responseDestination is notcharacteristicsrelationrelationpatternexpecting the data(variation)(variation)Preserve payloadwithin a certainbetweenbetweencontenttimeinformationinformationPreserve payloadentities of theentities of thecontentstreamstreamConversationalpattern (stringentand low delay)Example of thevoicestreaming videoWeb browsingbackgroundapplicationdownload ofemails
For each of these traffic classes, a list of QoS attributes can be defined as shown in the following table. If the QoS attributes are met, it is ensured that the message is perceived by the end user with the required quality. The QoS attributes are negotiated between the different elements of the communication chain (UE, RNC, CN elements) during the setup of a connection and depend on the type of service requested and the capabilities of the different nodes. If one of the QoS attributes is not met, the end user will certainly remark a degradation of the communication (e.g. voice deformation, connection blank, etc).
Traffic classConversationalStreamingInteractiveBackgroundclassclassclassclassMaximum bitrateXXXXDelivery orderXXXXMaximum SDUXXXXsizeSDU formatXXinformationSDU error ratioXXXXResidual bit errorXXXXratioDelivery ofXXXXerroneous SDUsTransfer delayXXGuaranteedXXbit-rateTraffic handlingXpriorityAllocation/XXXXRetention prioritySource statisticsXXdescriptorSignallingXIndication
During a Radio Access Bearer (RAB) assignment procedure, the RNC receives the parameters of the RAB to be established and in particular its QoS attributes. The CN initiates the procedure by sending a RAB ASSIGNMENT REQUEST message to the RNC. The message contains the IE “RAB Parameters”, which comprises all necessary parameters for RABs including QoS attributes.
Upon reception of the RAB ASSIGNMENT REQUEST message, the UTRAN executes the requested RAB configuration. The CN may indicate that RAB QoS negotiation is allowed for certain RAB parameters and in some cases also which alternative values to be used in the negotiation.
The general idea behind the RAB QoS negotiation is to provide a solution in case a user is asking for a service with specified QoS requirements, but for some reasons (e.g. resources are not available) the system cannot meet the requirements precisely. In such situation a negotiation of certain RAB parameters (QoS attributes) like guaranteed bit-rate or maximum bit-rate is allowed by the CN in order to provide the user at least a connection with compromised QoS attributes instead of leaving the user without service. RAB establishment and QoS negotiation are types of Iu admission control that is carried out in the C-RNC.
The admission control mentioned in the section with respect to the functionality split for E-DCH above refers to the admission control to the Serving Radio Network Subsystem. The resources of the Serving Radio Network Subsystem are thereby controlled by C-RNC. Iu admission control refers to the admission control to the Radio Access Network and is a function of the S-RNC.
As indicated above, in an UMTS radio access network insufficient information is available in the C-RNC for the purpose of admission control and congestion control of scheduled data.
For admission control of scheduled data by C-RNC it is necessary to gather the information in C-RNC from Node B about resource consumption (Layer-1 information) for given level of satisfaction of QoS requirements (Layer-2 information). This Layer-2 information is currently not available in the C-RNC. If already admitted guaranteed bit-rate (GBR) users have satisfactory performance in terms of QoS, an additional user may be admitted.
For congestion control of scheduled data by C-RNC it is necessary to gather the information from Node B about current consumption of resources (Layer-1 information) for given level of satisfaction of QoS requirements (Layer-2 information) as agreed with the S-RNC during call admission control so that these C-RNC may invoke certain actions to honor these requirements.
As a part of congestion control, C-RNC may reconfigure resources assigned to the E-DCH of a particular user (e.g. ‘Total Power’) or it may wish to pre-empt the traffic over given logical channel/MAC-d flow and switch it to legacy dedicated channel. Same as for admission control, this information is currently not available in the C-RNC.
The main problem is that Layer-2 information is missing in the C-RNC. QoS control for scheduled data cannot be properly exercised without this information.
However, given that High Speed Downlink Packet Access (HSDPA), Multimedia Broadcast Multicast Service (MBMS) and High Speed Uplink Access (HSUPA) will probably be deployed in respective temporal order in radio access network thus imposing additional requirements on transport network capacity and especially on “last mile” connection (i.e. Iub in legacy UTRAN). For example, Iub will not be optimized for multicast transmission (at least in Release 6 framework) meaning that point-to-multipoint MBMS Radio Bearer will be mapped to a multiplicity of Iub transport connections of point-to-point type. Depending on capital expenditure for “last mile” when deploying new features in the access network, larger or lower delays or more or less frequent congestion occurrences on this interface are possible. Therefore, minimizing possible Iub load may serve as a design constraint to the problems as identified above.